The use of fluoropolymers, particularly polytetrafluoroethylene ("PTFE"), as anti-drip and flame retardant additives in thermoplastic resin compositions is known. Since direct incorporation of a fluoropolymer into a thermoplastic resin matrix tends to be difficult, fluoropolymer-containing anti-drip and flame retardant additives typically take the form of a fluoropolymer that has been pre-blended in some manner with a small amount of a second polymer. For example, U.S. Pat. No. 5,521,230 discloses steam precipitation of an aqueous dispersion of PTFE and a polycarbonate resin to form a PTFE concentrate for use as a drip inhibitor additive in thermoplastic resin composition. U.S. Pat. No. 4,579,906 discloses melt blending a PTFE-thermoplastic resin powder with a thermoplastic resin matrix, e.g., a SAN or ABS matrix, and other flame retardant additives to produce a flame retardant thermoplastic resin composition. The PTFE-thermoplastic resin powder of the '906 patent is made by co-coagulating an aqueous PTFE dispersion that is stabilized with an ethoxylated nonyl phenol surfactant with an aqueous styrene-acrylonitrile ("SAN") resin emulsion or an aqueous acrylonitrile-butadiene-styrene ("ABS") resin emulsion and then precipitating and drying the co-coagulated PTFE-thermoplastic resin composition to provide a PTFE-thermoplastic resin powder.
While techniques for incorporating fluoropolymers into flame retardant thermoplastic resin compositions are known, further improvements are desirable. Known techniques typically employ aqueous fluoropolymer dispersions, which are typically stabilized with an ethoxylated nonyl phenol surfactant. Use of ethoxylated nonyl phenol surfactants is under scrutiny from the perspective of environmental safety and alternative approaches to stabilizing aqueous fluoropolymer dispersions are highly desirable. The amount of fluoropolymer that can be incorporated in a fluoropolymer-thermoplastic resin composition by co-coagulation techniques is limited and would be desirable to provide a fluoropolymer-containing additive having a higher fluoropolymer content. Co-coagulated fluoropolymer-thermoplastic resin compositions tend to be very difficult to handle due to clumping and poor flowability and it is correspondingly difficult to incorporate such additives uniformly and reproducibly into a thermoplastic resin composition. Non-uniform distribution of fluoropolymer additive within a thermoplastic resin composition may result in, e.g., surface imperfections, such as e.g., streaking and splay, and in inconsistent combustion performance, such as, e.g., uneven shrink rates and dripping. A fluoropolymer-thermoplastic resin additive that is in the form of a free-flowing powder would be highly desirable from both the perspective of material handling and the perspective of improving the uniformity and reproducibility of the thermoplastic resin compositions made therefrom.